The present invention relates to telephony systems and, more particularly, to a circuit and method for inserting an alternating current (AC) ringing signal onto the subscriber loop side of the telephone line to a telephone.
In most, if not all, present day U.S. telephone systems a ringing relay is utilized which, when enabled from the telephone central office, inserts a ringing signal onto the subscriber loop path that includes the subscriber telephone. The ringing signal is typically at a nominal frequency of 20 hertz and between 70-130 volts RMS. The ringing signal is used to energize the clapper-bell of a telephone to indicate that a telephone call is being received, as is understood. During the ringing cycle a closed conduction path is provided for a ringing generator, located at the central office, through the telephone and the ringing relay to earth ground. As the ringing relay is energized the normal signal conversion path is open circuited from the subscriber loop path and, at the same time, the ringing relay connects the Ring side of the subscriber loop directly to the ringing generator while grounding the Tip side to earth ground through a series connected resistor. This action provides a low impedance path for the AC ringing signal through the subscriber telephone.
The use of a ringing relay suffers from several disadvantages. The relays are relatively expensive and bulky. Further, the relay contacts tend to wear out and/or corrode over time and therefore need to be continually replaced. In addition, the relays can cause electrical transients to be generated during normal operation. These transients can be coupled or induced into adjacent telephone cable pairs to produce noise on adjacent subscriber loop paths.
Thus, there is a need for a solid state ringing insertion circuit to overcome the above disadvantages. Such a circuit would be relatively less expensive than ringing relays conventionally used.